The present invention relates to a guided beam type of flat display device wherein at least one and preferably each of a plurality of electron beams are scanned over a different area portion of an image screen. The invention relates particularly to a scan deflection structure for scanning each of the beams in one of its two orthogonal scan directions within its area portion of the screen.
As shown and described in the application for U.S. Patent of C. H. Anderson et al, Ser. No. 615,353, filed Sept. 22, 1975, entitled "Guided Beam Flat Display Device" now Pat. No. 4,028,562 issued June 7, 1977 one structure which has been proposed for a large area screen flat display device comprises a thin box-like envelope with one of the large sides thereof constituting a faceplate on which a phosphor screen is disposed. Within the envelope are a plurality of spaced, parallel support walls perpendicularly disposed to and between the large sides of the envelope forming a plurality of parallel channels. The support walls are typically of glass and serve to support the opposite large-area envelope walls against the compressive atmospheric pressure. Across one end of the channels is a gun structure which directs at least one electron beam along each of the channels. In each of the channels is a beam guide which confines the electron beam in the channel and guides the beam along the length of the channel. The beam guide also includes means for deflecting the electron beam out of the beam guide at selected points along the beam guide. The beams in all of the channels are simultaneously deflected out of the beam guides toward the phosphor screen at each of the selected points. Along the opposite walls at each side of each channel are deflection electrodes whereby each beam in each channel can be deflected transversely across the channel to achieve a line-by-line scanning of the phosphor screen.
In this display device the transverse movement of the beams is achieved by applying a positive potential to the deflection electrode at one side of each of the channels, e.g. the right side, and a negative potential to the deflection electrode at the other side of each of the channels, e.g. the left side. These potentials are generally in relation to the potential applied to the phosphor screen. This causes all of the beams in all of the channels to be deflected toward the deflection electrode which is at the positive potential, i.e. toward the right side of the channels. The potentials applied to the deflection electrode are then varied to cause each of the beams to simultaneously scan across the portion of the screen which transverses the respective channel.
A problem with this manner of operating the display device is that there is created a relatively high capacitance across each of the support walls between the deflection electrodes on opposite sides of each of the support walls. This capacitance is a result of the different potentials applied to the deflection electrodes on each side of each side wall, i.e. positive on one side and negative on the other, the thinness of the walls and the dielectric constant of the glass material of the walls. The high capacitance is undesirable since it produces a relatively high power dissipation by the scanning circuitry as a result of relatively high charging and discharging current needed to establish the required deflection voltage therefrom.